Most modern wind turbines are controlled and regulated continuously with the purpose of ensuring maximum power extraction from the wind under the current wind and weather conditions, while at the same time ensuring that the loads on the different components of the wind turbine are at any time kept within acceptable limits.
In order to avoid excessive loads or undesirable wear on the wind turbine components, most modern wind turbines are controlled such as to be shut down if the wind speed exceeds some predetermined value often called the stop wind or cut-off wind.
The stopping process may simply be to stop the wind turbine e.g. by pitching the blades to their feathering position, or by adjusting some other active aerodynamic devices for changing the aerodynamic surfaces of the blades such as flaps, adjusting the power, and/or adjusting the rotational speed of the rotor.
However, in a wind power plant where a number of wind turbines are shutting down at high wind speeds in order to protect themselves may lead to sudden and significant drops in the power production, which may cause severe problems for the grid. Also, shutting down and restarting of wind turbines at high wind speeds in general can produce significant wear on the turbine components.
As an alternative to shutting down at wind speeds above a certain stop wind, it is possible to de-rate the power of the wind turbines as a function of wind speed so that the power is reduced for increasing wind speeds. Such a method of operation is described in EP 0847496 where both the power and the rotational speed of the wind turbine are decreased continuously with increasing wind speeds. However, such methods may in some situations and especially in turbulent and extreme wind conditions yield a very fluctuating de-rating of the wind turbines and may therefore be insufficient and in some cases even inadequate to protect the turbines.